TappingMode imaging is a key advance in atomic force microscopy (AFM) of soft, adhesive or fragile samples. This patented technique allows high resolution topographic imaging of sample surfaces that are easily damaged, loosely held to their substrate, or otherwise difficult to image by other AFM techniques. Specifically, TappingMode overcomes problems associated with friction, adhesion, electrostatic forces, and other difficulties that can plague conventional AFM scanning methods. The technique has proven extremely successful for high resolution imaging of a wide variety of samples including:

· silicon wafer surfaces

· thin films

· metals and insulators

· photoresist

· polymers

· biological samples

· and numerous others.

AZoM - The A to Z of Materials - TappingMode image of purified collagen monomer and oligomer molecules without telopeptides.

Figure 1. TappingMode image of purified collagen monomer and oligomer molecules without telopeptides.

TappingMode makes imaging these surfaces routine in ambient air or fluids and represents a significant advance in AFM technology.

Two conventional scanning modes — contact mode and non-contact mode — have been used for some time with varying success for a range of materials.

Each has limitations which are discussed below and contrasted with TappingMode scanning.
Conventional Methods

In conventional contact mode AFM (Figure 2), the probe tip is simply dragged across the surface and the resulting image is a topographical map of the surface of the sample. While this technique has been very successful for many samples, it has some serious drawbacks. The dragging motion of the probe tip, combined with adhesive forces between the tip and the surface, can cause substantial damage to both sample and probe and create artifacts in image data.

AZoM - The A to Z of Materials - Comparison of contact mode, non-contact mode and TappingMode scanning techniques. Contact mode imaging (left) is heavily influenced by frictional and adhesive forces which can damage samples and distort image data. Non-contact imaging (center) generally provides low resolution and can also be hampered by the contaminant layer which can interfere with oscillation. TappingMode imaging (right) eliminates frictional forces by intermittently contacting the surface and oscillating with sufficient amplitude to prevent the tip from being trapped by adhesive meniscus forces from the contaminant layer. The graphs under the images represent likely image data resulting from the three techniques.

Figure 2. Comparison of contact mode, non-contact mode and TappingMode scanning techniques. Contact mode imaging (left) is heavily influenced by frictional and adhesive forces which can damage samples and distort image data. Non-contact imaging (center) generally provides low resolution and can also be hampered by the contaminant layer which can interfere with oscillation. TappingMode imaging (right) eliminates frictional forces by intermittently contacting the surface and oscillating with sufficient amplitude to prevent the tip from being trapped by adhesive meniscus forces from the contaminant layer. The graphs under the images represent likely image data resulting from the three techniques.

Under ambient air conditions, most surfaces are covered by a layer of adsorbed gases (condensed water vapor and other contaminants) which is typically several nanometers thick. When the scanning tip touches this layer, capillary action causes a meniscus to form and surface tension pulls the cantilever down into the layer (Figure 3). Trapped electrostatic charge on the tip and sample can contribute additional adhesive forces. These downward forces increase the overall force on the sample and, when combined with lateral shear forces caused by the scanning motion, can distort measurement data and cause severe damage to the sample, including movement or tearing of surface features.

AZoM - The A to Z of Materials - In contact AFM, electrostatic and/or surface tension forces from the adsorbed gas layer pull the scanning tip toward the surface.

Figure 3. In contact AFM, electrostatic and/or surface tension forces from the adsorbed gas layer pull the scanning tip toward the surface.

Some researchers have overcome the problems associated with the adhesive forces by operating AFMs with the sample immersed in fluid. When scanning in fluids, the overall forces in contact mode are lower than in ambient air because the fluid layer/meniscus is not present and electrostatic forces can be dissipated or screened. However, because hydrated samples are often substantially softer than dried samples, tracking forces can still cause reduced image quality and sample damage due to deformation and/or movement of the sample by the scanning probe. In addition, many samples, such as semiconductor wafers, can not practically be immersed in fluid.

An attempt to avoid this problem is the non-contact mode in which the probe is held a small distance above the sample (Figure 2). Attractive Van der Waals forces acting between the tip and the sample are detected, and topographic images are constructed by scanning the tip above the surface. Unfortunately, the attractive Van der Waals forces from the sample are substantially weaker than the forces used by contact mode — so weak in fact that the tip must be given a small oscillation so that AC detection methods can be used to detect the small forces between tip and sample. The attractive forces also extend only a small distance from the surface, where the adsorbed gas layer may occupy a large fraction of their useful range.

Hence, even when the sample tip separation is successfully maintained, non-contact mode provides substantially lower resolution than either contact or TappingMode. In practice, the probe is frequently drawn to the sample surface by the adsorbed gases’ surface tension, resulting in unusable data and sample damage similar to that caused by the contact technique. In addition, the non-contact mode is generally impractical for routine scanning in fluids because the Van der Waals forces are now even smaller, a substantial limitation for biological samples in particular.
TappingMode Imaging in Air

TappingMode imaging overcomes the limitations of the conventional scanning modes by alternately placing the tip in contact with the surface to provide high resolution and then lifting the tip off the surface to avoid dragging the tip across the surface. TappingMode imaging is implemented in ambient air by oscillating the cantilever assembly at or near the cantilever’s resonant frequency using a piezoelectric crystal. The piezo motion causes the cantilever to oscillate with a high amplitude (the “free air” amplitude, typically greater than 20nm) when the tip is not in contact with the surface. The oscillating tip is then moved toward the surface until it begins to lightly touch, or “tap” the surface. During scanning, the vertically oscillating tip alternately contacts the surface and lifts off, generally at a frequency of 50,000 to 500,000 cycles per second. As the oscillating cantilever begins to intermittently contact the surface, the cantilever oscillation is necessarily reduced (Figure 4) due to energy loss caused by the tip contacting the surface. The reduction in oscillation amplitude is used to identify and measure surface features.